Gas turbine engines are widely used to power aircraft. As is well known, the engine basically provides propulsive power by generating a high velocity stream of gas which is exhausted rearwards through an exhaust nozzle. A single high velocity gas stream is produced by a turbojet gas turbine engine. More commonly nowadays however two streams, a core exhaust and a bypass exhaust, are generated by a ducted fan gas turbine engine or bypass gas turbine engine.
The high velocity gas stream produced by gas turbine engines generates a significant amount of noise, which is referred to as exhaust or jet noise. This noise is generated due to the high velocity of the exhaust stream, or streams, and the mixing of the streams with the surrounding atmosphere, and in the case of two streams, as the bypass and core streams mix. The degree of noise generated is determined by the velocity of the stream and how the streams mix as they exhaust through the exhaust nozzle.
Increasing environmental concerns require that the noise produced by gas turbine engines, and in particular aircraft gas turbine engines, is reduced and there has been considerable work carried out to reduce the noise produced by the mixing of the high velocity gas stream(s). A large number of various exhaust nozzle designs have been used and proposed to control and modify how the high velocity exhaust gas streams mix. With ducted fan gas turbine engines particular attention has been paid to the core stream and the mixing of the core and bypass exhaust streams. This is because the core stream velocity is considerably greater than the bypass stream and also the surrounding atmosphere and consequently the core exhaust stream generates a significant amount of the exhaust noise. Mixing of the core stream with the bypass stream has also been found to generate a significant proportion of the exhaust noise due to the difference in velocity of the core and bypass streams.
One common current exhaust nozzle design that is widely used is a lobed type nozzle which comprises a convoluted lobed core nozzle as known in the art. However, this adds considerable weight, drag, and cost to the installation and nowadays short bypass nozzles are favoured with which the lobed type core nozzles are less effective and are also more detrimental to the engine performance than when used on a long cowl arrangement.
An alternative nozzle design that is directed to reducing exhaust noise is proposed and described in GB 2,289,921. In this design, a number of circumferentially spaced notches, of various specified configurations, sizes, spacing and shapes, are provided in the downstream periphery of a generally circular core exhaust nozzle. Such a nozzle design is considerably simpler to manufacture than the conventional lobed designs. This prior proposal describes that the notches generate vortices in the exhaust streams. These vortices enhance and control the mixing of the core and bypass streams which it is claimed reduces the exhaust noise.
Model testing of nozzles similar to those described in GB 2,289,921 has shown that significant noise reduction and suppression can be achieved. However the parameters and details of the design proposed in GB 2,289,921 are not optimal and there is a continual desire to improve the nozzle design further.
A further design, and that of the present Assignee, is proposed in UK Application GB 0025727.9. This application discloses trapezoidal shaped tabs disposed to the axially rearward exhaust ducts of the bypass and core and which are inclined radially inward to impart vortices to the exhaust streams.
However, the main requirement of reducing exhaust noise is during aircraft take-off and landing. At higher altitudes where the majority of the duration of the flight is, exhaust noise is not a problem. It is therefore not necessary to have noise reduction means operational at higher altitudes especially when one considers the noise reduction means inherently introduces aerodynamic inefficiencies.
The use of shape memory materials (“SMMs”) to achieve movement in engineering applications has been known since at least the 1970s. SMMs are a class of materials that exhibit large changes in property over a relatively small temperature range (or ranges) and are generally stable at other temperatures. The most common is currently a Binary Nickel Titanium alloy, which is often used in a ternary state when Copper, Hafnium or Palladium is added. Other metal systems are known, such as those based on Copper or Iron. Certain Ceramics and polymers also exhibit similar properties. The majority are reversible, i.e. the properties return to original conditions on returning to the original temperature, although hysteresis is often present. Some are non-reversible, i.e, once the change with temperature has occurred, they do not return to their original condition. Systems that use triggers other than temperature are also known, for example magnetic (in magnetic (or ferromagnetic) shape memory alloys (MSM/FSM)), chemical, and light.
On heating through a pre-determined temperature range, the SMM undergoes a phase change. This is manifested as a change in properties. In its simplest form, a material can be “plastically” deformed when cold and will recover to its original state on heating. If an external load, such as a spring, is applied, the change in properties is conventionally considered a change in modulus of the material. When acting against the spring or load, the change in modulus leads to two equilibrium positions as the compression expansion of the SMM changes. More than two conditions could be useful, but suitable materials are not currently known.
The vast majority of applications and research have used SMM wires or thin strips attached to or embedded within a structure. The use of discrete wires severely limits the volume of material which can be utilised, which limits the total energy that can be expended and the integrity of the structure. Intimate contact between the SMA and parent structure (as in U.S. Pat. No. 6,718,752) severely limits the overall strain that can be achieved because the parent engineering material preferably is subject to a strain similar to that of the SMM. The optimum strain for current SMMs is around 2% whereas conventional structural materials must work below around 0.3% strain. This leads to severe compromises between the deflection that can be achieved and the stiffness of the structure as it opposes external (operational) loads. To achieve large deflections it must be thin, but this results in a low stiffness such that the SMM can be bent easily by external loads such as air, liquid etc. The choice must be made between either low stiffness and high deflection or high stiffness and low deflection.